(3ej) Assessment of Fouling in Native and Surface-Modified Water Filtration Membranes

Fouling is a major obstacle to implementation of polymeric membranes in water purification applications, causing deterioration of membrane performance and, consequently, increases in operational expenses.  Surface modification is one way of alleviating membrane fouling.  Most porous membranes are made of hydrophobic polymers, promoting the adhesion of foulants via strong hydrophobic-hydrophobic interactions.  Hydrophilization of the membrane surface tends to reduce fouling because a layer of water molecules becomes tightly bound to the hydrophilic surface, acting as a buffer against foulant ingress.  Dopamine was recently found to non-selectively deposit from buffered alkaline solution to form “polydopamine” on virtually any surface, rendering the surface very hydrophilic after only a few minutes of contact with dopamine solution.  The chemical structure of polydopamine, which was previously ambiguous, was investigated by many techniques, including solid state NMR, FTIR, UV/vis and powder X-ray spectroscopies.  Polydopamine was also used as a platform for the molecular conjugation of other anti-fouling materials, such as poly(ethylene glycol), to the membrane surface.

This work has focused on the evaluation of fouling behavior in native and surface-modified commercial membranes.  Water filtration membranes can be operated in two ways: constant permeate flux with variable transmembrane pressure (TMP) or constant transmembrane pressure with variable permeate flux.  Membrane fouling may be measured using either of these two methodologies.  A constant permeate flux crossflow system was carefully constructed and used to challenge native and surface-modified membranes with an oil/water emulsion.  The critical flux—the flux below which minimal fouling occurs—was determined by a flux-stepping technique during constant flux crossflow filtration.  The membrane performance during fouling was evaluated at fluxes higher than and lower than the critical flux.  Membrane resistance evolution during fouling is compared for constant-flux and constant-TMP operation.  The efficacy of surface modifications in reducing biofouling was also studied.  Many evaluations of anti-biofouling behavior rely on short-term tests of protein or bacterial adhesion.  The results of such experiments were compared to those obtained for comparatively long-term operation under hydrodynamic conditions relevant to industrial module operation.